They have become my favorite fish after my girlfriend decided she wanted me to set her up a DP tank. Their names are Beavis and Butthead and they remind me of little puppy dogs, constantly begging for food and craving attention at the front of the tank! Awesome fish.
As other have said, species only tanks for puffers are best. They are nippy with each other as well, and some must be housed alone. I do keep 3 DPs and an oto together with no problems (yet), the oto has some good hiding spots though, and the tank is very heavily planted. I would not trust them with other types of fish.